The invention relates to an apparatus for sputtering, especially reactive sputtering, with preferably a magnetron cathode, with a moving target, especially a rotating target.
In sputtering processes, high-power sputtering apparatus are used in practice in which a magnetic field in front of the cathode increases the probability of the collision of the particles and hence of their ionization. The heart of these high-power sputtering apparatus is the so-called magnetron cathode.
Such a magnetron cathode is described, for example, in German patent 2417288, which corresponds to U.S. Pat. No. 4,166,018.
In it is shown a cathode sputtering apparatus with a high sputtering rate, with a cathode having on one of its surfaces the material to be sputtered and to be deposited on a substrate, with a magnet system arranged such that magnetic lines of force issuing from the sputtering surface and returning thereto form a discharge area which has the shape of a closed loop, and with an anode disposed outside of the paths of the sputtered material moving from the sputtering surface to the substrate.
In the above-cited patent it is proposed that the cathode surface to be sputtered and facing the substrate to be sprayed be flat, that the substrate be moved over the planar sputtering surface parallel to the latter and close to the discharge area, and that the magnet system producing the magnetic field be disposed on the side of the cathode facing away from the planar sputtering surface.
In the prior art are also sputtering apparatus with a rotating magnetron cathode. The prospectus of Airco Coating Technology, a division of the BOC Group, Inc., with the identification ACT10110K988, hereinafter called "Airco Prospectus," describes the construction and the operation of such a sputtering apparatus, having a rotating magnetron cathode. As it appears from the illustration and the text of the Airco Prospectus, it is, strictly speaking, only the cylindrical or tubular-shaped target that rotates. The stationary magnet unit of the magnetron cathode is located in the interior of the target. Important components of such a known magnetron cathode include (see the Airco Prospectus)--in addition to the rotating cylindrical target and the stationary magnet unit--the target drive system, a water cooling system, a vacuum chamber in which the rotating target and the substrate, among other things, are located, plus a power supply unit for the cathode. In practice, the target material is applied as a coating on a tube consisting of copper, for example. The system, consisting of a target coating and copper tube, rotates in front of the ionized plasma.
The prior art also includes U.S. Pat. No. 4,356,073, which corresponds to European Patent 0070899. In this patent is described an apparatus for sputtering thin films of a selected coating material onto substantially planar substrates, consisting of an evacuable coating chamber, a cathode mounted horizontally in this coating chamber, with an elongated, cylindrical tube element on whose outer surface a layer of the coating material to be sputtered is applied, and magnet means which are disposed in this tube element in order to provide a sputtering zone extending longitudinally therefrom.
U.S. Pat. No. 4,356,073 discloses rotating this tubular element about its longitudinal axis in order to bring various parts of the coating material into a sputtering position opposite the above-mentioned magnet means and within the above-mentioned sputtering zone, and through means situated in the coating chamber for the horizontal support of the substrates and for transporting the latter past the magnet means so that these substrates will receive the sputtered material.
There is also known Patent 161040 of the German Democratic Republic, though it does not relate to a rotating target. This last-named patent discloses an apparatus for the prevention of undesired discharges in reactive DC cathode sputtering, especially reactive DC high-rate sputtering, for the purpose of making electrically highly insulating coatings by using conventional plasmatron sputtering apparatus and a target edging applied to the target and reaching to a point close to the discharge.
In the last-named patent it is proposed that the target edging consist of an electrically insulating material of the smallest possible thickness, and that between the target edging and the target a gap-like cavity be provided whose width is no more than a few tenths of the average free travel length of the sputtered particles. The points of contact of the target edging with electrically conductive parts of the structure are at least twenty times the width of the gap-like cavity away from the edge of the target, and the target edging is covered at least in part by a shield whose distance from the target edging is of the order of magnitude of the width of the gap-like cavity.
In rotating tubular targets the central area is eroded by the plasma while no erosion occurs at the extremities. In the sputtering of certain materials, such as SiO.sub.2 sputtered layers additionally form on the tube ends during the sputtering process.
During the sputtering process, the rotating target is eroded by the above-described processes so that the middle portion has a smaller diameter, while the tube ends have a larger diameter. In a number of applications, the tube ends even become additionally thickened by the described growth of a coating of sputtered material on the tube ends.
The processes described above lead to a heightened danger of undesired arcing between the target and the inside wall of the chamber, between target and substrate, and between the target and the coating consisting of sputtered material that has already grown on the substrate. This danger of arcing exists especially in the case of the following sputtering materials: SiO.sub.2, Al.sub.2 O.sub.3, ZrO.sub.2, ZnO, Ta.sub.2 O.sub.5.